The calculators on this site also run as three small free JSON APIs. They use the same math as the pages (same tests), so you can put 2026 self-employment tax, quarterly estimates, the QBI deduction, platform fees or GSA per diem rates into your own app, spreadsheet, invoice tool or AI agent. Each API answers GET with query parameters or POST with a JSON body, and sends CORS headers so it works from the browser. No API key is needed. Hit / on any of them to get a list of endpoints and a working example.

API Base URL What it covers
US Freelancer Tax API (2026) https://us-freelancer-tax-api.narwhalapis.workers.dev Self-employment tax, quarterly estimated tax, 2026 due dates, IRS mileage (split-year rates), home office (simplified vs Form 8829), SEP IRA / Solo 401(k) limits, QBI (Section 199A) deduction
Seller Fee API https://seller-fee-api.narwhalapis.workers.dev Fee, net payout and effective rate for Gumroad (direct, high volume, Discover), Etsy, Stripe and PayPal (each payment type), plus the price you need to keep a target amount
US Per Diem API (GSA FY2026/FY2027) https://us-per-diem-api.narwhalapis.workers.dev GSA lodging and M&IE rates by ZIP, city or state, and trip totals with the 75% first and last day rule

All three APIs are also listed on RapidAPI with a free plan: US Freelancer Tax API on RapidAPI, Seller Fee API on RapidAPI and US Per Diem API on RapidAPI.

Freelancer tax examples

Self-employment tax on $80,000 of net profit:

curl "https://us-freelancer-tax-api.narwhalapis.workers.dev/v1/se-tax?net_profit=80000"

You get net earnings from self-employment ($73,880), Social Security ($9,161.12), Medicare ($2,142.52), the total self-employment tax ($11,303.64) and the deductible half.

Quarterly estimated tax for a single filer, sent as JSON:

curl -X POST "https://us-freelancer-tax-api.narwhalapis.workers.dev/v1/estimate" \
  -H "content-type: application/json" \
  -d '{"net_profit": 80000, "filing_status": "single"}'

The answer has total federal tax ($16,647.47 in this example), the share of profit to set aside (20.81%) and the four quarterly payments with their 2026 due dates.

QBI (Section 199A) deduction:

curl "https://us-freelancer-tax-api.narwhalapis.workers.dev/v1/qbi?qbi=80000&taxable_income=70000&filing_status=single"

Here the 20% QBI amount is $16,000, but the taxable income cap (20% of $70,000) limits the deduction to $14,000. Other endpoints: /v1/figures (every 2026 figure with its IRS source), /v1/due-dates, /v1/estimate/batch, /v1/mileage, /v1/mileage/rates, /v1/home-office and /v1/retirement.

Seller fee examples

What Gumroad takes on a $20 direct sale:

curl "https://seller-fee-api.narwhalapis.workers.dev/v1/fee?platform=gumroad&amount=20"

Fee $3.38 (10% + $0.50, plus card processing), net $16.62. To compare Gumroad, Etsy, Stripe and every PayPal payment type at once, sorted by what you keep, call /v1/compare?amount=20. To find the price that leaves you $20 after fees, call /v1/price?platform=gumroad&target=20, which gives $23.89. /v1/platforms lists every platform and the rates it uses, each with a link to the official fee page.

Per diem examples

GSA rates for a ZIP code:

curl "https://us-per-diem-api.narwhalapis.workers.dev/v1/rates?zip=10001"

Trip total for New York City, Nov 10–12, 2026:

curl "https://us-per-diem-api.narwhalapis.workers.dev/v1/trip?zip=10001&start=2026-11-10&end=2026-11-12"

That’s 2 nights at the $358 lodging maximum ($716) plus $230 of M&IE ($69 on each 75% travel day and $92 on the full day), so $946 in total. /v1/meta shows the fiscal years the data covers, and /v1/mie gives the M&IE breakdown table.

Use them from an AI assistant (MCP)

If you’d rather ask an AI assistant than call HTTP, the same math ships as two free MCP servers on npm: freelance-tax-mcp (self-employment tax, quarterly estimates, hourly rate, QBI and more) and seller-fees-mcp (platform fees and target prices). Run them with npx -y freelance-tax-mcp or npx -y seller-fees-mcp.

Limits and fine print

  • The APIs run on a free hosting plan. Please keep request rates reasonable and use the batch endpoints for bulk work.
  • Results are estimates for planning. They follow published IRS, SSA, GSA and platform rates, which are listed in each response, but they are not tax advice. Check the official source before you file.
  • Platform fees change. Every seller fee response includes rates_as_of and the source URL.

Disclosure: these APIs, MCP servers and calculators are our own, and they’re free. We don’t earn anything from the links on this page, and none are affiliate links. Not affiliated with the IRS, GSA or any of the platforms listed. More guides: Gumroad developer guides.